Printed circuits were originally made by taking a flat substrate of insulating material and, with a printing press, applying conducting material to the insulation. Later it was realized that it would be easier to make printed circuits by coating the entire insulated board with a film of metal and then etching away those areas where a conductor is not desired. Under this method, the printed circuit board was made by taking a metal coated board and applying to it a pattern of acid-resistant material, called resist. Then the board, clad with metal and resist, was dipped in an acid bath, removed after an appropriate time and washed, leaving on the board only the desired conducting areas, the acid having removed those areas of the metal which were left unprotected by the resist.
An alternative technology for fabricating circuits, known as thick film technology, has been used principally from the early 1960's. It is a process for creating an entire circuit by direct printing. In thick film technology, an insulating substrate, usually ceramic, is used as a base upon which a conductive ink is deposited. A conductive ink can be created by suspending silver particles, or the like, in a carrier. It is this conductive ink which is printed directly on the circuit. In a similar manner, a resistor can be applied to the circuit by using a resistive material suspended in the carrier in place of silver and printing this resistive material (not to be confused with "resist") directly onto the base and creating the circuit, commonly by silk screening. There are similar techniques for creating capacitors and inductors in this manner. The technology is referred to as thick film because of the depth of material that is supplied to the substrate. This depth is relatively thick (often hundreds of microns). Thick film technology has limited application to integrated circuit manufacture and is not suitable for active matrix arrays, in which elements must be thin and small.
The technology that was developed for creating integrated circuits, which was perfected after the 1960's, is photolithography in which there is no physical contact between the integrated circuit being created and the machinery creating it. After the conducting or semiconducting surface is applied, a photosensitive resist layer is applied on top. The photosensitive resist layer is then exposed to light in a selective manner determined by a photographic mask pattern. The mask pattern is created by graphically drawing a mask, photographing it and using the resulting negative to project light onto the photosensitive resist layer. This exposes the resist in certain places and, depending on whether it is a positive or negative resist, there is a chemical change during the exposure which makes the resist either impervious or not impervious to acid. The circuit is then bathed in acid to etch away all but the desired circuit and, in this manner, it is possible to create an integrated circuit without any physical contact with the surface being dealt with. Photolithography permits the creation of extremely small circuit elements because the wavelength of visible light permits high resolution. By the mid 1970's, the microelectronics industry was almost exclusively using photoresist technology in the production of microelectronic circuits. In a 1975 book, Photoresist: Materials And Processes, W. S. DeForest stated, "Today the entire semiconductor industry is dependent on the use of photoresists for the manufacture of their devices and circuits. Numerous photoetching steps are employed sequentially in order to form the various active and passive components of the circuit."
The current practice in the industry is Si integrated circuits and this has carried over into the production of active matrix circuits in which devices are constructed from amorphous films in contrast to the single crystal material generally used for integrated circuits. These films exhibit low carrier mobility necessitating small line widths, short device channels and great alignment accuracies.
The need in conventional integrated circuit technology for small line widths and extremely complex masks has motivated development of successively more sophisticated lithography and etching techniques, including x-ray lithography and electron and ion beam etch. These techniques require concomitantly slower and more expensive equipment. For example, ion beam etching requires the circuit to be placed in a vacuum chamber, which is expensive and precludes rapid manufacture. Direct printing is thought by the art to be unsuitable for integrated circuit manufacture because:
1. the required narrow line widths cannot be achieved;
2. the substrate cannot be registered accurately through multiple passes to allow formation of multiple layers; and
3. the pressure required to print on an integrated circuit would damage layers previously fabricated.
An active matrix is one type of integrated circuit in which elements may be selectively caused to emit or modulate light. Active matrix technology is a technology used in, for example, flat panel television screens in which a semiconductor matrix is created; each individual cell of which can be activated by an electrical signal sent down the matrix in the x and y directions. The production of such an active matrix has been very expensive due to the use of conventional stepping engines that step and repeat a particular pattern many times for each layer of each matrix being manufactured.
High-definition television (HDTV) will begin to be available in commercial products. Some test programs of HDTV have already been aired. Furthermore, several companies have announced a new cathode-ray tube (CRT) displays for HDTV. Flat type displays which utilize active matrix technology should be most favorable for HDTV because CRT displays for HDTV's can be very large, cumbersome and heavy. Recently, liquid crystal display (LCD) technology has brought about a large advance in flat panel displays. Thin film transistor (TFT) technology has realized high resolution and large area LCD's. Furthermore, 10" diagonal LCD's with 640.times.480 pixels are already commercially available.
Manufacturers are carrying out a great deal of research aimed at further definition and larger panel sizes. HDTV is fundamentally a large screen medium. Therefore, a size of more than 40" diagonal and full color display is preferably required. TFT-LCD's, or active matrices, are currently made by a photolithographic technique, however, it is very difficult to apply this technique to fabricate a large active matrix LCD for HDTV because the size of the matrix for HDTV becomes very large.
One of the important problems in such flat panel fabrication is the process of patterning fine components on a large scale with good registration and reproduction. The steps of photolithography have been involved in the conventional method and require bulky and expensive equipment with low throughput. If the step and repeat process can be replaced by a single printing technique having processing abilities for large areas with a high throughput, substantial contributions may be obtained for mass production of large size panels with low initial investment.
A considerable number of individual process steps required in the fabrication of flat panels have been heretofore required in photolithography. An abbreviated list of the steps involved in each photolithographic step is as follows:
1. spin-on, or otherwise deposit, photoresist film;
2. bake;
3. align in exposure system;
4. expose pattern on resist;
5. for larger area substrates, step to another region and repeat exposure to cover total area;
6. develop;
7. rinse and dry; and
8. inspect panel with possible reworking.
These steps must be repeated for each successive mask layer, of which there are usually four or more. Note that the present photolithographic process requires a considerable amount of handling of individual substrates. Steps 4 and 5 must be performed individually and become an increasingly important bottleneck as display areas increase. A photoresist pattern representing a small subset of the entire layer (a 2".times.2" photoresist pattern, for example) is projected onto an area and this must be repeated several times to encompass the entire layer of the active matrix. The stepping engine must be precise for proper alignment with each step. The expensive part of this process is the time involved and the expense of the machine involved in doing the stepping and the repeating of the photoresist patterns.